The present invention relates to a fluorescent indicator tube driving circuit and, more particularly, to a circuit for driving a multi-digit fluorescent indicator tube.
Conventional fluorescent indicator tubes are characterized in that they comprise a plurality of heaters(cathodes), anodes, and grids contained within a vacuum tube. Hot electrons are emitted from the cathodes by heating the heaters and are accelerated by the grids using a pulse voltage of about 20-50 v. The accelerated electrons collide against fluorescent elements on the segmented anodes to emit fluorescence. Thus, the tubes can display numerals, characters, symbols, ets., in display digits, in accordance with the shapes of the anodes.
Conventionally, the heaters connected to a multi-digit fluorescent indicator tube are in series connected. In the respective display digits, a desired electron accelerating voltage between a heater and an anodesegment, and a bias voltage between a grid and the heater can be obtained by adding a heater voltage to an anode power source and a grid bias power source, respectively. The heater voltage is related to the respective display digits as a reference point of each of the power sources.
When the heaters are heated using a DC voltage, display brightness inevitablly varys between the upper digits and the lower digits. To correct this problem conventionally, the heaters are heated using an AC voltage to render the display brightness appearance uniform.
However, this approach is disadvantageous in that an AC voltage is needed for heating the heaters.
Therefore, it is desired to enable the heaters to be heated using a DC voltage.